To achieve desired motor movement, the phase of currents flowing through motor windings must be synchronized. In DC motors employing rotor magnets, currents of the stator windings must be phased properly so that the angular position of the magnetic field generated by the stator currents has the proper orientation with respect to the magnetic fields associated with the rotor magnets. Key to proper phasing is knowledge of the angular position of the rotor and its magnets relative to that of the stator. In DC motors with commutators (i.e., brushes), proper phasing is done automatically. In contrast, brushless DC motors require detectors to detect changes in the magnetic fields associated with changes in the angular position of the rotors.
Magnetic field detectors often incorporate a Hall effect device mounted on a printed circuit board. Orientation of the Hall effect device is important because the device is most sensitive to magnetic fields perpendicular to one pair of its surfaces. To that end, many motors mount a Hall effect device on a printed circuit board that is perpendicular to the rotor axis. To be properly oriented to sense the magnetic field generated by the rotor magnets, the Hall effect device is mounted on its edge perpendicular to the circuit board, usually occupying a region between the rotor magnets and the stator so as to permit the device to detect the magnetic fields produced by the rotor magnets.
Surface mounting circuitry produces significant cost advantages in terms of speed of assembly. However, surface mounting the Hall effect device on a circuit board located perpendicular to the rotor axis both 1) removes the device from the region of maximum magnetic field and 2) orients the device in a manner that is least sensitive to the magnetic fields generated by the rotor magnets. In other words, mounting a Hall effect device in this manner does not permit its surfaces to be perpendicular to the magnetic field, thus minimizing its functionality.